Blood-sucking flies may have been to blame for the HIV epidemic being unleashed on humans, scientists suggest. Many Aids researchers believe the HIV virus jumped species from chimpanzees to humans at some point in the first half of the 20th century.

They think humans were first exposed when simian immunodeficiency virus (SIV), the monkey version of HIV, got into open wounds of game hunters in west or central Africa.

However, German scientists think stable flies may be responsible for HIV invading humans, according to an article in New Scientist magazine.

The message is to practice safe sex and not to worry about being bitten by flies.

Professor David Mabey, communicable disease expert Most blood-sucking insects pose no risk of passing on HIV, including mosquitoes, which inject saliva through one tube and suck up blood through another.

However, stable flies, which bite humans, could be an exception and are known to transmit equine leukaemia virus between horses.

When feeding, they scrape skin to make a wound, suck up blood and regurgitate some on the skin next time they feed.

Any viruses in the regurgitated blood can invade the body through the wounds made by the flies.

Unlike other blood-sucking insects that regurgitate blood, the stable fly does not digest the blood it regurgitates.

Gerhard Brandner of the University of Freiburg, said: "The anterior part of the mid-gut where the regurgitate is kept is just for storage and is free of digestive enzymes.

"That's our key result, and is a precondition for transmission of Hiv."

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